This invention relates to testing electrical traces, such as on a substrate, such as a circuit board, for characteristics such as opens, shorts, neck-downs, or improper etching; and more specifically relates to a test method using photoelectric effect.
In the manufacture of electronic components, the packaging density has increased considerably, resulting in extremely narrow and thin traces disposed on both sides of the substrates with numerous connections from one side to the other. Fabrication of such fine traces is difficult such that defects are more common. Therefore, testing the quality of fine traces on both sides and connections from one side to the other of a substrate has become increasingly more important.
Most conventional methods of trace testing involve physically contacting the trace with one or two test probes. The physical placement accuracy and the physical size of test probes limit their use in testing in mass. Many current traces are so small or densely packed that they can only be connected with a physical probe individually with a very time consuming and uneconomical process. Even if contact probes are available and can be used in a production mode, the act of contacting, more specifically the force required to make a good electrical contact is high enough to inflict permanent damage to thin traces, rendering them useless.
Therefore there has been a need for a test apparatus and method for opens, shorts, neck-downs, or improper etching in which the trace is not physically contacted and which is not unduly time consuming.
This invention is a tester for electrical traces such as on a circuit board, and the preferred embodiment generally comprises a laser producing an ultraviolet beam, a vacuum chamber, an electrode circuit including electrodes and corresponding electronics including ammeters for measuring photoelectron flow between traces and electrodes, a controller, laser beam optics, an image acquisition system, and a pair of broadband UV lights.
The board containing traces under test is disposed in the vacuum chamber at lowered pressure with grid electrodes lying over the trace area on each side of the board. Electrode electronics selectively maintain a known potential on each electrode. The exact location of traces are determined by an image acquisition system.
The traces are initialized to a known voltage such as by: (1) the induced high voltage method by first applying a relatively high positive voltage to one electrode and a high negative voltage to the other electrode, then reversing the polarity of the voltages; (2) simultaneous photoelectron effect illumination of electrodes by setting both electrodes to a known positive voltage and irradiating electrodes, board and traces by the broadband UV electromagnetic sources; or (3) laser method by setting electrodes to a known voltage and dithering the laser beam so as to strike a portion of electrode and a trace. Shining the laser beam on a location so as to liberate photoelectrons is referred to as xe2x80x9cinterrogatingxe2x80x9d.
Continuity between two points on a trace is determined by interrogating the first location until it is charged to a known voltage and then by interrogating the second location.
Shorts between traces can be determined by interrogating a first trace until it is charged to a known voltage and then interrogating the second trace.
Other features and many attendant advantages of the invention will become more apparent upon a reading of the following detailed description together with the drawings wherein like reference numerals refer to like parts throughout.